Added some number formatting capability

Hi, davidchambers.

I added some functionality to string-format that allows you to specify number sign, float precision, and field padding.

Your tests pass and I added a few of my own.  Any interest in merging this?

It now has the regexp-fu to parse everything in the [python string format mini-language](http://docs.python.org/2/library/string.html#format-specification-mini-language), although it doesn't implement absolutely everything yet.  I implemented what I needed for the project I was working on.  Perhaps someone else or a future me will be motivated to finish the job.

If you do decide to merge, you will want to change the part in the readme about this being a fork.

Cheers,

Ted Scharff